1. Field of the Invention
The invention relates to a method and apparatus for the stepwise forming of a series of troughs of varying depth and width into a metal strip with the aid of a number of sets of bottom (male) and top (female) dies sequentially applied to the metal strip to gradually form the troughs. At least the first of the sets of bottom and top dies serves to pre-form the metal strip which is, in additional forming steps, further impressed until the desired trough is embossed in the metal strip.
2. Description of the Prior Art
It is difficult to impress troughs into thin metal foil strips since they are very susceptible to cracking. German Pat. Nos. 1,162,316; 1,198,314 and 1,217,911 show how cracking of the metal foil can be avoided if the impressions are made carefully and sufficient foil material is available in the imprint area. In order to have sufficient foil material available it has, for example, been proposed to prepleat the strip in the area in which a trough is to be impressed.
German publication DT-AS No. 1,602,485 proposes to imprint, during the initial impressing steps, a trough which is deeper than the final trough and to leave a gap between the bottom and top dies in their engaged positions. This, especially, facilitates the forming of decorative pleats in the bottom wall of the trough. As a result of the right-angled side walls of the troughs and a uniform cross-section, the consumption of material is naturally constant over the full length of the trough. However, if the cross-section is not uniform over the length of a trough (i.e. the width of the metal strip) that is if the trough has no uniform depth or width the strip material consumption is different at the opposite sides of the strip and the strip tends to deviate from the rectilinear advancing movement through the successive forming stages which may result in faulty imprints.